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  <div class="question_difficulty">
   难度：Medium
  </div>
  <div>
   <h1 class="question_title">
    77. Combinations
   </h1>
   <p>
    Given two integers
    <em>
     n
    </em>
    and
    <em>
     k
    </em>
    , return all possible combinations of
    <em>
     k
    </em>
    numbers out of 1 ...
    <em>
     n
    </em>
    .
   </p>
   <p>
    <strong>
     Example:
    </strong>
   </p>
   <pre>
<strong>Input:</strong>&nbsp;n = 4, k = 2
<strong>Output:</strong>
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]
</pre>
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  <div>
   <h1 class="question_title">
    77. 组合
   </h1>
   <p>
    给定两个整数
    <em>
     n
    </em>
    和
    <em>
     k
    </em>
    ，返回 1 ...
    <em>
     n
    </em>
    中所有可能的
    <em>
     k
    </em>
    个数的组合。
   </p>
   <p>
    <strong>
     示例:
    </strong>
   </p>
   <pre><strong>输入:</strong>&nbsp;n = 4, k = 2
<strong>输出:</strong>
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]</pre>
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